1. Field of the Invention
This invention is directed toward a method of testing blowout preventers (BOP) located at a well head to prevent unrestricted flow of gas and or oil from a well during an emergency situation.
Oil and Gas Exploration risk management includes the ability to control subsurface pressures which may be encounter during drilling operation. The primary mechanism utilized by operators to control downhole pressures is the hydrostatic pressure as a result of the drilling fluid contained within the wellbore. The drilling fluid is engineered and formulated to a density that provides a hydrostatic pressure inside of the wellbore that is greater than the formation pressure being drilled. In the majority of drilling operations, the hydrostatic control of wellbore pressure is adequate. However, from time-to-time the operator may encounter a higher than expected formation pressure where there is not adequate hydrostatic pressure to control the wellbore pressure. During these times the operator relies on a series of mechanical controls to stabilize the wellbore and prevent a “Blow Out”. A blow out is the uncontrolled release of fluid or gas from the wellbore. This event is extremely dangerous and therefore must be avoided if at all possible. The primary mechanical control device utilized by operators to control wellbore pressure is the Blowout Preventer (BOP) assembly. The BOP assembly consists of multiple sealing and shearing devices that are hydraulically actuated to provide various means of sealing around the drill string or shearing it off entirely, completely sealing the wellbore. It is essential that the BOP assembly operate as designed during these critical operations. Therefore it is a regulatory requirement to test the functionality and the integrity of the BOP assembly before starting drilling operations and at specific events during the drilling operations.
2. Description of Related Arts Invention
The BOP assembly test is a series of pressure tests typically at a minimum of two pressure levels, low pressure and high pressure. During the pressure test, intensification fluid from a high pressure intensification pump is introduced into the closed BOP assembly in a volume sufficient to cause the internal pressure within the closed BOP assembly to rise to the first pressure test level. Once the first pressure test level is established the high pressure intensification pump is isolated from the closed BOP assembly and the pressure is monitored for a specified time period. During the monitoring phase the pressure decay is determined and compared to the pressure decay specification. A typical specification for compliance allows for a pressure decay rate of no more than 5 psi/minute or 25 psi total over the entirety of the five minute test.
Measuring leak rate utilizing the indirect result of pressure decay, while widely accepted, is problematic. This is especially apparent when performing BOP assembly tests offshore in deeper waters. In a typical offshore configuration the BOP assembly will be located at the sea floor. The distant between the BOP assembly and the drilling platform at the surface can reach upwards of 10,000 feet. The BOP assembly is connected to the drilling platform via tubular pipe sections typically referred to as the “riser assembly”. The drill string is a series of tubular pipes attached to the drilling platform at one end and the drill bit or service assembly at the opposite end. The drill string is positioned within the riser assembly. During a typical BOP hydrostatic test the drill string and riser assembly are filled with drilling fluid. The BOP is configured for the applicable hydrostatic test which acts to close off or seal the drill string. A high pressure intensification pump, typically the cement pump, is aligned so as to add additional drilling fluid, or other suitable intensification fluid, via the open end of the drill string drill string at the drilling platform, in a volume sufficient to cause the pressure within both the BOP assembly and the drill string to rise to the appropriate test pressure. The volume of drilling fluid required to raise the pressure within the BOP assembly and the drill string to the applicable level is related to the compressibility of the drilling fluid within the BOP assembly and drill string as well as the intensification fluid. For example: a typical offshore BOP assembly and the drill string might require approximately 100 bbls of drilling fluid to completely fill the area between the BOP assembly and the drilling platform. Typical drilling fluids used in offshore drilling have a compressibility factor of approximately 0.0035/1000 psi. A typical BOP assembly test pressure might be 5,000 PSI. Therefore in this example the additional volume of intensification fluid required to raise the internal pressure of the BOP assembly and the drill string is 1.75 bbls. If the required test pressure of the BOP assembly is 10,000 psi, the additional volume of intensification fluid required to raise the internal pressure of the BOP assembly and the drill string is 3.5 bbls.
In most cases a high pressure reciprocating intensification pump is utilized to pump the required additional drilling fluid into the BOP assembly and drill string. The action of pumping intensification fluid from an ambient pressure to a significantly high pressure, sometimes in excess of 20,000 psi creates heat. The heat is principally generated by mechanical inefficiencies of the intensification pump and the compressive strain of the drilling fluid. The temperature rise subsequent to the intensification pump is a function of the pressure differential and the volume of drilling fluid pumped. In some cases the temperature of the intensification fluid can rise as much as 40 deg F. The temperature rise has a significant effect on the volume/pressure relationship within the BOP assembly and the drill string due to the thermal coefficient of expansion of the intensification fluid. The thermal coefficient of expansion of intensification fluids and drilling fluids varies greatly but a typically might have a thermal coefficient of expansion of approximately 0.0003 per degree Fahrenheit. Therefore if during the pressurization phase of the BOP pressure test, the intensification fluid temperature is raised approximately 30 degrees F. by the intensification pump, the volume will increase approximately 0.009 or approximately 1%.
Referring to the previous example above where approximately 3.5 bbls of intensification fluid was added to the BOP assembly and the drill string to raise the pressure to approximately 10,000 psi will equate to a pressure increase slope of approximately 2850 psi/bbl of intensification fluid added. Referring to the previous example above where a 30 degree F. increase in intensification fluid temperature results in approximately a 1% increase in volume will further equate to 0.035 bbls (3.5×0.01=0.035).
Subsequent to pumping, the heated intensification fluid will cool at a rate defined by the general thermal conductivity of the surrounding environment. As the intensification fluid cools there is a corresponding reduction in volume equal to the previous thermally induced volume increase. The reduction in volume causes the pressure to decrease at a rate approximately equal to the pressure slope previously described. In this example the decrease in pressure would be approximately 100 psi over the period of time necessary for the temperature of the intensification fluid to return to ambient. This period of time can be as little as 5 minutes to as much as 20 minutes. During this time the pressure decay rate exceeds the limit of 5 psi/minute. Therefore the pressure test of the BOP assembly cannot begin until the pressure decay has stabilized at a rate less than 5 psi/minute. This period of time is known within the industry as “waiting on a flat line”. Once the pressure decay stabilizes at or below 5 psi/minute the BOP pressure test can begin.
It would be desirable to eliminate the affects of the temperature increase in the pressurizing fluid. This would increase the accuracy of the test and also reduce the amount of time required for each test segment. This would greatly decrease the cost of BOP testing.
Additionally, in certain environments where the ambient temperature is cold, it may be necessary to further heat rather than cool the intensification fluid so that it is approximately equal to the temperature of the BOP assembly or other closed hydraulic systems. In certain testing situations the BOP assembly may not contain fluid or it may be partially or completely filled with fluids.
In any case the temperature of the BOP assembly is measured and the intensification fluid is heated or cooled as necessary to match the temperature of the BOP assembly or other closed hydraulic system.